Point-to-multipoint data transmission

ABSTRACT

The initial transmission of data packets in a point-to-multipoint communication network may often be delayed when retransmissions of erroneously decoded data packets are required. According to an aspect of the present invention, point-to-multipoint data transmission and retransmission of erroneously decoded data from a transmitting station to a plurality of receiving stations is performed, where retransmitted data is sent via at least one communication channel, which is different from the one used for the initially transmitted data. Advantageously, due to this there may be no need for further communication between the transmitting station and the receiving stations. Furthermore, according to an aspect of the present invention, the initial transmission of data packets is not delayed by retransmissions.

The present invention relates to point-to-multipoint or multicast datatransmission. More particularly, the present invention relates to amethod for performing point-to-multipoint data transmission, acommunication system, a transmitting station and a receiving station.

A point-to-multipoint communication network basically comprises atransmitting station and a plurality of receiving stations. Data istransmitted by a multicast signal (i.e. the sender once sends thissignal, which is decoded by all receiving stations) from thetransmitting station to the receiving stations via a multicastcommunication channel established by means of a communication medium.Upon decoding the data, each receiving station may send a feedbacksignal back to the transmitting station, the feedback signal comprisinginformation with respect to whether the received data could be decodederror-free or not. By receiving the feedback information, thetransmitting station may decide whether a retransmission for arespective data packet is required or not.

The expression “retransmission for a (data) packet” is used here inorder to state that the retransmitted bits do not necessarily form anexact copy of the bits, which were sent in the initial transmission ofthe packet. Retransmissions can be either full copies of the initiallytransmitted packet, or a retransmission can contain different data, e.g.only additional parity bits, which together with the received bits ofthe initially transmitted packet are considered in the decoding process(this is an example for non-self-decodable incremental redundancy). Ageneral example for so-called self-decodable redundancy would be thatthe retransmission contains all systematic bits, but a different set ofparity bits compared to the initial transmission.

In contrast to this, “retransmission of data” is used here to denoteboth “a retransmission of a (data) packet” (i.e. an exact copy of thedata packet is retransmitted) and “a retransmission for a (data packet)(i.e. an exact copy of the data packet is retransmitted or the bitscarried in the retransmission differ from the bits of the initialtransmission).

Various multicast communication networks are known.

U.S. Pat. No. 4,285,064 relates to time division multiple accesssatellite communications.

U.S. Pat. No. 5,457,808 discloses a point-to-multipoint communicationnetwork, which comprises a transmitting station and a plurality ofreceiving stations for transmitting a multicast signal from thetransmitting station to the receiving stations and which is capable ofretransmitting at least a portion of the multicast signal from thetransmitting station to a failing station, which fails among thereceiving stations to receive the portion. This point-to-multipointcommunication network includes multicast communication means forestablishing communication channels between said transmitting stationand said receiving stations and for transmitting a multicast signal oversaid communication channels and retransmitting means for selecting oneof said communication channels between said transmitting station andsaid failing station and for transmitting said portion over said one ofthe communication channels.

It is an object of the present invention to provide for an improvedpoint-to-multipoint data transmission.

According to an exemplary embodiment of the present invention as setforth in claim 1, the above object may be solved by a transmission ofdata from the transmitting station to the plurality of receivingstations via a first communication channel, a switching of the firstchannel in the transmitting station from the first communication channelto a second communication channel and a retransmission of the data viathe second communication channel.

Throughout this document, the term “switching from a first communicationchannel to a second communication channel” includes that after thisswitching both the first and the second communication channel areavailable, i.e. the transmitting station sends data on both, and areceiving station may decode both the first and the second communicationchannel, if required because it needs retransmissions for a previouslysent packet. In other words, switching from a first communicationchannel to a second communication channel in the context of thisdocument means,

-   -   for the transmitting side, that the second communication channel        is switched on, in addition to the first communication channel,        while keeping the first communication channel and e.g.        continuing sending new data via this first communication channel    -   for the receiving side, that reception or decoding of the first        communication channel is continued, while the receiving side in        addition starts to receive or decode the second communication        channel.

In other words, according to this exemplary embodiment of the presentinvention, a transmission method for point-to-multipoint or multicasttransmission of data is provided, the method being capable ofretransmitting transmitted data, wherein the data has been transmittedfrom the transmitting station to a group of receiving stations via afirst communication channel and wherein the data is retransmitted fromthe transmitting station to the group of receiving stations via thesecond communication channel. Simultaneously with the retransmission onthe second communication channel, new data may be transmitted via thefirst communication channel.

According to this exemplary embodiment of the present invention, thismay be achieved by performing a first channel switching in thetransmitting station from the first communication channel to the secondcommunication channel after transmission of the data and beforeretransmission for the data. It may be stated that first and secondcommunication channels form a point-to-multipoint channel set used forperforming multicast transmission.

Advantageously, according to this exemplary embodiment of the presentinvention, the retransmission of the data does not interfere with thetransmission of other data, since the retransmission takes place on adifferent communication channel.

According to another exemplary embodiment of the present invention asset forth in claim 2, the data is sent as a data packet and a channelswitching is performed in at least one second receiving station of theplurality of receiving stations from the first communication channel tothe second communication channel. The at least one second receivingstation, which performs the second channel switching, erroneouslydecoded the data packet and, for this reason, performs the secondchannel switching. First and second channel switchings are performed inaccordance with a first and a second switching scheme, wherein the firstswitching scheme and the second switching scheme correspond to eachother.

Advantageously, the method according to this exemplary embodiment of thepresent invention provides for a retransmission for a data packet, whichhas been erroneously decoded by at least one second receiving station,without feedback communication between all the second receiving stationsand the transmitting station. This omission of feedback signaling savesvaluable resources, i.e. channel load and time and saves energy whichmay be in particular advantageous for wireless networks.

According to another exemplary embodiment of the present invention asset forth in claim 3, the delay time interval between the data packettransmission and the data packet retransmission is predefined and setforth in the first and second switching schemes. Due to this, the atleast one second receiving station “knows” when the retransmission is tobe expected. No signaling is necessary since the delay time is set forthin the second switching scheme. The predefined delay time intervalbetween the transmission of the data packet and the retransmission forthe data packet may provide enough time for performing the first andsecond channel switching in the transmitting station and the pluralityof receiving stations respectively.

According to another exemplary embodiment of the present invention asset forth in claim 4, a data packet transmission of a respective datapacket is carried out on the first communication channel, if the datapacket is a new data packet which has not yet been transmitted.According to an aspect of this exemplary embodiment of the presentinvention, the first communication channel is always and only used forprimary or initial transmission of data packets, i.e. data packets,which are transmitted for the first time. Therefore, no retransmissionfor a data packet will be carried out on the first communicationchannel, while retransmissions are still ongoing on the secondcommunication channels, which means that a receiving station, whichlistens exclusively to the first communication channel, usually onlyreceives primary or initial data, i.e. data which is transmitted for thefirst time, and does not receive secondary data, i.e. data which hasalready been transmitted for a first time and is now beingretransmitted. First or initial transmission of data is thereforeperformed via the first communication channel and there is nointerference with the secondary or retransmitted data or data packets.Also, due to this, advantageously, in case a receiving station could notdecode the initial data, it knows that it may receive the retransmissionon the second communication channel.

According to another exemplary embodiment of the present invention asset forth in claim 5, a receiving station from the plurality ofreceiving stations which erroneously decoded a data packet, sends anegative acknowledgement message to the transmitting station via a thirdcommunication channel. Furthermore, the transmitting station will notperform a retransmission for the data packet if the transmitting stationdid not receive the negative acknowledgement message from the receivingstation. According to this exemplary embodiment of the presentinvention, a retransmission for a respective data packet only occurs ifa receiving station erroneously decoded the respective data packet andthe retransmission for the respective data packet does not occur ifevery receiving station of the plurality of receiving stations decodedthe respective data packet successfully. It should be noted thatalternatively, a receiving station of the plurality of receivingstations may send a positive acknowledgement message to the transmittingstation via a third communication channel, if the respective receivingstation decoded the data packet successfully, and that, when thetransmitting station receives a positive acknowledgement from all thereceiving stations from the plurality of receiving stations, noretransmission for the data packet is performed by the transmittingstation.

According to another exemplary embodiment of the present invention asset forth in claim 6, there is provided a plurality of secondcommunication channels, via which retransmissions of a data packet areperformed. Furthermore, according to this exemplary embodiment of thepresent invention, each of the retransmissions of the plurality ofretransmissions is carried out via a different communication channel ofthe plurality of second communication channels, as defined in the firstand second switching schemes.

In other words, a retransmission for a respective data packet may beperformed more than once, wherein the first retransmission for therespective data packet is performed via a second communication channelof the plurality of second communication channels, and wherein a secondretransmission for the respective data packet is performed via anothersecond communication channel of the plurality of second communicationchannels, etc. For each of the retransmissions of the plurality ofretransmissions, a different communication channel of the plurality ofsecond communication channels is used. Which channel is used for whichretransmission is set forth in the first and second switching schemes.

Advantageously, according to an aspect of the present invention, areceiving station, which erroneously decoded a respective data packet,may switch to a communication channel of the plurality of secondcommunication channels, according to the second switching scheme, inorder to receive a retransmission for the respective data packet. If theretransmission for the respective data packet failed or if theretransmitted respective data packet has been erroneously decoded by therespective receiving station, the respective receiving station mayswitch to another, different communication channel of the plurality ofsecond communication channels, according to the second switching scheme.The transmitting station also switches to the other communicationchannel of the plurality of second communication channels according tothe first switching scheme. After that, a second retransmission for therespective data packet is performed via this other communication channelof the plurality of second communication channels, etc.

Advantageously, following this protocol, there is no need forcommunication between the transmitting station and the plurality ofreceiving stations, since each receiving station knows at all times,which data packet is transmitted or retransmitted via whichcommunication channel of the first communication channel and theplurality of second communication channels, as defined in the first andsecond switching schemes.

According to another exemplary embodiment of the present invention asset forth in claim 7, retransmissions of different data packets areperformed simultaneously via different communication channels of theplurality of second communication channels, and simultaneously with thetransmission of new data packets via the first communication channel.Advantageously, this may allow to increase the data transmission rateand reduce the delay.

According to another exemplary embodiment of the present invention asset forth in claim 8, the plurality of receiving stations within reachof the transmitting station is partitioned into groups of receivingstations with comparable or similar channel conditions of the first andsecond communication channels. According to this exemplary embodiment ofthe present invention, point-to-multipoint or multicast data packettransmission is performed for each group individually. Advantageously,by grouping receiving stations with comparable or similar channelconditions and by performing point-to-multipoint data packettransmission for each group individually, the channel load for aparticular group may be reduced, since no retransmission of data may berequired for that particular group, and the resulting delay getssmaller, the better the channel conditions of a group are.

According to another exemplary embodiment of the present invention asset forth in claim 9, transmission of a data packet and retransmissionfor the data packet is performed via a wireless communication link.According to an aspect of this exemplary embodiment of the presentinvention, the transmitting station and the plurality of receivingstations may be connected by a communication medium, which may include atransponder on board an artificial satellite. But it should be noted,that any other form of wireless communication link may be used,according to the present invention.

According to another exemplary embodiment of the present invention asset forth in claims 10 and 11, the method is an extension of the HybridAutomatic Repeat Request (hereinafter referred to as “HARQ”) protocol inthe Universal Mobile Telecommunications System (hereinafter referred toas “UMTS”) or is applied in the context of 3rd Generation PartnershipProject Frequency Division Duplex (hereinafter referred to as “3GPPFDD”) mode. The 3GPP FDD mode provides a High Speed Downlink SharedChannel (hereinafter referred to as “HS-DSCH”), which can be used withcode multiplexing, i.e. within one slot it is possible to transmit datato the same user using several codes. Instead of using several codes toincrease the data rate of within one slot, each code channel may be usedas one of the channels of a point-to-multipoint channel set. By applyingHARQ type II and III, the retransmissions may carry only incremental(e.g. non-self-decodable) redundancy, so that the interference generatedby the additional code channels used for the retransmissions becomessmaller than that of an additional code channel, which carriesself-decodable redundancy, since incremental redundancy may need asmaller number of bits than self-decodable redundancy. In addition, thespreading factor on the code channel, that is used for an initialtransmission in parallel to retransmission for earlier packets ondifferent parallel code channels, may be increased so that for thisinitial transmission a smaller packet is carried in order to furtherreduce the overall interference resulting from the new transmission inparallel to retransmissions for earlier packets, which interferenceimpacts neighboring cells and is generated by the cell, in which thisHS-DSCH is modified so that point-to-multipoint transmission ispossible. Currently point-to-multipoint transmission is not possibleaccording to 3GPP TS 25.321 “3rd Generation Partnership Project;Technical Specification Group Radio Access Network; Medium AccessControl (MAC) Protocol Specification (Release 5) Version 5.5.0”.

According to other exemplary embodiments of the present invention as setforth in claims 12, 13 and 14, a communication system is provided forperforming point-to-multipoint data packet transmission from atransmitting station to a plurality of receiving stations. Thetransmitting station is adapted to transmit a data packet from thetransmitting station to the plurality of receiving stations via a firstcommunication channel and the transmitting station is adapted to performa first channel switching in the transmitting station from the firstcommunication channel to a second communication channel and toretransmit the data packet via the second communication channel.Furthermore, the plurality of receiving stations is adapted to receivethe data packet transmitted from the transmitting station and to performa second channel switching in at least one second receiving station ofthe plurality of receiving stations from the first communication channelto the second communication channel. The at least one second receivingstation, which performs the second channel switching, erroneouslydecoded the data packet and thus needs a retransmission of the data.

It may be seen as the gist of an exemplary embodiment of the presentinvention that any initial data is transmitted via an n-th channel andeach i-th retransmission of the data for receiving stations which couldnot decode the preceding transmission is retransmitted via an (n+i)-thchannel. Due to this, the receiving stations know where to find(receive) the transmitted data. Hence, the signaling between thetransmitting station and the receiving stations may be reduced or evencompletely omitted. Advantageously, this reduces interference and maysave energy, in particular in wireless transmission systems.

These and other aspects of the present invention will become apparentfrom and elucidated with reference to the embodiments describedhereinafter.

Exemplary embodiments of the present invention will be described in thefollowing, with reference to the following drawings:

FIG. 1 shows a schematic representation of a communication system forperforming point-to-multipoint data packet transmission according to anexemplary embodiment of the present invention.

FIG. 2 shows a simplified timing chart, depicting a transmission and aplurality of retransmissions of a data packet from a transmittingstation to two receiving stations in the communication system depictedin FIG. 1 in accordance with an exemplary embodiment of the presentinvention.

FIG. 3 shows a flowchart of an exemplary embodiment of a methodaccording to the present invention.

FIG. 4 shows a flowchart of another exemplary embodiment of a methodaccording to the present invention.

FIG. 5 shows a schematic representation of the transmission of datapackets and retransmission for these data packets from a transmittingstation according to an exemplary embodiment of the present invention.

FIG. 6 shows a schematic representation of the transmission of datapackets and retransmission for these data packets from a transmittingstation according to an exemplary embodiment of the present inventiontaking into account longer delay faced by the transmitting station forreceiving feedback information from some or all of the receivingstations.

For the description of FIGS. 1 to 6, the same reference numerals areused for the same or corresponding elements.

The communication system for performing point-to-multipoint data packettransmission from a transmitting station to a plurality of receivingstations, as depicted in FIG. 1, comprises a transmitting station 1 anda plurality of receiving stations 2, 3, 4, 5 and 6. The transmittingstation 1 performs a point-to-multipoint or multicast transmission ofdata to the receiving stations 2, 3, 4, 5 and 6 via a communication link7. According to an exemplary embodiment of the present invention,communication link 7 is a wireless communication link. Data may betransmitted from the transmitting station 1 to the receiving stations 2,3, 4, 5 and 6. Transmission of data from the transmitting station 1 tothe receiving stations 2, 3, 4, 5 and 6 is performed in terms of apoint-to-multipoint or multicast transmission, using a multicasttransmission channel for the transmission of data packets. Thecommunication system may be a cellular mobile communication system suchas a GSM-network or a UMTS network. It could also be a wireless LocalArea Network (LAN).

FIG. 2 shows a simplified timing chart of an exemplary embodiment of amethod of operating the communication system of FIG. 1 according to thepresent invention.

In FIG. 2 and in FIG. 5, capital letters A, B, C, D and E refer to aninitial transmission of data packets A, B, C, D and E, respectively.Capital letters A′, B′, C′, D′ and E′ refer to a first retransmissionfor data packets A, B, C, D and E, respectively. Accordingly, A″, B″,C″, D″ and E″ refer to a second retransmission for data packets A, B, C,D and E, respectively, etc.

Transmitting station 1 transmits via a point-to-multipoint transmissionon a first multicast communication channel 21 a data packet A to aplurality of receiving stations 2, 3. Receiving station 2 and receivingstation 3 do not decode the transmitted data packet A successfully, asindicated by thunderbolts 25 and 26. Then, a first channel switching isperformed in the transmitting station 1 according to a first switchingscheme, meaning that for the first retransmission A′ for the data packetA, a second multicast communication channel is used, while the firstmulticast communication channel 21 is still used for initialtransmissions of new data packets. Furthermore, a second channelswitching is performed in each of the receiving stations 2 and 3,according to a second channel switching scheme, wherein the firstswitching scheme corresponds to the second switching scheme. This secondchannel switching means that the receiving stations 2 and 3 continue todecode or receive the first multicast communication channel 21, andstart to decode or receive the second multicast communication channel22.

According to an aspect of this exemplary embodiment of the presentinvention, the channel switching may be performed during a delay timeinterval between the transmission of data packet A and a successivefirst retransmission A′ for the data packet A. The delay time intervalis set forth in the first and second switching schemes. After the delaytime interval, during which a channel switching may be performed intransmitting station 1 and receiving stations 2 and 3 from the firstchannel 21 to second channel 22, the transmitting station 1 sends theretransmission A′ for the data packet A to the receiving stations 2 and3 via the second channel 22, while a new data packet B is sentsimultaneously via the first channel 21. Unfortunately, both receivingstations 2 and 3 are not able to decode the retransmitted data packet A′successfully, as indicated by the thunderbolts 27 and 28. Then, a thirdand fourth channel switching is performed in the transmitting station 1and the two receiving stations 2 and 3 respectively. The third channelswitching in the transmitting station 1 is performed according to thefirst channel switching scheme, and the fourth channel switching in thereceiving stations 2 and 3 is performed according to the second channelswitching scheme. According to the first and second channel switchingschemes, the transmitting channel of the transmitting station 1 (forsending the second retransmission for data packet A) and the receivingchannels of the two receiving stations 2 and 3 (for receiving the secondretransmission for the data packet A) are switched to a third channel23, while the first channel is still available for transmission of a newdata packet, and the second channel is available for transmitting aretransmission for the data packet B. Again, the third and the fourthchannel switching is performed during a delay time interval between thefirst retransmission A′ for the data packet A and a secondretransmission A″ for the data packet A, as well as between thetransmission of B and the first retransmission B′ for B. After the thirdand fourth channel switching, a second retransmission A″ for the datapacket A is performed by the transmitting station 1 via a thirdmulticast channel 23, while simultaneous sending of the retransmissionB′ is performed via the second multicast channel 22, and the sending ofa new data packet C is performed via the first multicast channel 21.Again, receiving station 2 erroneously decodes the data packet, asindicated by thunderbolt 29, but receiving station 3 successfullydecodes the data packet A″, as indicated by checkmark 30. In a furtherstep, a fifth channel switching is performed in the transmitting station1 and a sixth channel switching is performed in the receiving station 2,both during a predefined delay time interval, as set forth in the firstand second switching schemes. According to the first and secondswitching schemes, both the transmitting station 1 and the receivingstation 2 switch channels for transmission respectively reception of thethird retransmission A′″ for data packet A to a fourth channel 24. Then,a third retransmission A′″ for the data packet A takes place, whilesimultaneously a possibly necessary second retransmission B″ for datapacket B is done via the third multicast channel 23, a possiblynecessary first retransmission C′ of data packet C is done via thesecond multicast channel 22, and a new transmission of a data packet Dis done via the first multicast channel 21. The retransmitted datapacket A′″ is only received and decoded by receiving station 2, not byreceiving station 3, since receiving station 3 decoded data packet A″successfully.

As may be seen from FIG. 2, the location or order of the retransmissionof the data is set forth in the first and second channel switchingschemes. These first and second channel switching schemes are known tothe transmitting station and the receiving stations. Due to this, incase a receiving station is unable to decode a data packet, it knowsfrom the second channel switching scheme where, i.e. at which channelthe next retransmission takes place. Hence, a complex notificationmessaging from the transmitting station to the receiving stations may beomitted. Furthermore, only those receiving stations switch to the secondchannels for decoding the retransmitted data which erroneously decodedthe precedent transmission (initial transmission). The receivingstations which could decode the initial transmission error free mayremain on the first channel. This may allow to avoid an unnecessary,energy consuming decoding of retransmitted data, which is not needed.

The flowchart depicted in FIG. 3 shows an exemplary embodiment of amethod for performing point-to-multipoint data transmission according tothe present invention as it may be implemented in the communicationsystem of FIG. 1. Transmitting station 1 transmits a first new datapacket 8 to a receiving station 2 via a first channel CH1. The receivingstation 2 fails to decode 9 the transmitted data packet successfully.Then, a channel switching 10, 11 is performed in the transmittingstation 1 and the receiving station 2, respectively. Both channelswitchings 10 and 11 are performed according to a respective first andsecond channel switching scheme. After this switching, the first channelCH1 is available for transmission in the transmitting station 1 (and forreception in the receiving station 2) of a second new data packet. Inthe next step, a retransmission 12 for the first new data packet isperformed (simultaneously with the transmission of the second new datapacket via CH1) by the transmitting station 1 via a second channel CH2,after which the receiving station 2 decodes the retransmitted datapacket successfully 13. Again, a channel switching 14 is performed inthe transmitting station 1, followed by a second retransmission 15 ofthe data packet via a third multicast transmission channel CH3. Thischannel switching 14, in addition, comprises that after this switching,CH1 is available in the transmitting station 1 for transmitting a thirdnew data packet, and CH2 is available in the transmitting station 1 fortransmitting a first retransmission for the second new data packet.However, since the decoding 13 of the first retransmission for the firstnew data packet done via CH2 has been performed successfully, no secondchannel switching involving CH3 is performed in the receiving station 2and therefore the second retransmission 15 for the first new data packetvia channel CH3 is not received in the receiving station 2 und thus doesnot need to be decoded. The receiving station 2 might perform a secondchannel switching involving CH2 so that it is available for reception ofthe first retransmission for the second new data packet, depending onwhether the initial or primary transmission of the second new datapacket was received error-free. This is not shown in FIG. 3.

Another receiving station might have requested the second retransmissionfor the first new data packet, and this receiving station then doesperform the second channel switching also involving CH3, so that in thisother receiving station CH1 is available for reception of the third newdata packet, CH2 is available for reception of the first retransmissionfor the second new data packet, and CH3 is available for reception ofthe second retransmission for the first new data packet.

As set forth so far, the mechanism for conveying retransmissions makesuse of a set of N+1 multicast channels, where the first channel carriesa new transmission of a data packet, and the i-th (i=2, . . . , N+1)channel in the set carries in a slot the (i−1)-th (i=2, . . . , N+1)retransmission for a packet.

FIG. 4 shows a flowchart of another exemplary embodiment of a method forperforming point-to-multipoint data transmission according to thepresent invention as it may be implemented in the communication systemof FIG. 1.

The transmission scheme depicted in FIG. 4 corresponds to thetransmission scheme depicted in FIG. 3, except that the receivingstation 2 may send a negative acknowledgement 16 to the transmittingstation 1, whenever the decoding 9 of a transmitted data packet 8 fails.For this, a feedback channel may be provided between the receivingstations and the transmitting station. The primary transmission 8 of thea first new data packet is performed via a first multicast channel CH1,from the transmitting station 1 to the receiving station 2. Afterrealizing that the decoding of the data packet transmission 8 failed,the receiving station 2 sends a negative acknowledgement NACK 16 back tothe transmitting station 1 via a channel CH10, which may be apoint-to-point feedback channel e.g. a random access channel, whichother receiving stations can also use in order to convey their feedbacksignaling. The transmitting station 1 receives the negativeacknowledgement NACK 17 and a first channel switching 10 is performed inthe transmitting station 1 according to the first channel switchingscheme. A second channel switching 11 is performed in the receivingstation 2 in accordance with a second channel switching scheme which maycoincide with the first channel switching. It should be noted that firstand second channel switching schemes may correspond to each other, inthe sense that the switchings follow each other, i.e. the transmittingstation is switched to the same channels as the receiving stations suchthat there is a time period where the transmitting stations and thereceiving station are switched to the same channel. After these channelswitchings 10 and 11,

-   -   CH1 is available (on the transmitting station) for transmission        (and on the receiving stations for reception) of the second new        data packet    -   CH2 is available (on the transmitting station) for transmission        (and on the receiving stations for reception) of the first        retransmission for the first new data packet.

It should be noted that each initial transmission of new data packetsmay always be carried out via the first communication channel. Everyrecipient or receiving station, which does not need a retransmission fora particular data packet, only listens to the first channel. If areceiving station fails to successfully decode a particular data packet,it may switch to the respective retransmission channel according to thesecond switching scheme. There is no need for communication between thetransmission station 1 and the receiving station 2 concerning thechannel number, which is used for a particular retransmission for a datapacket, since the time and the channel number of each retransmission ispre-set in the first and second switching schemes. This is particularlyimportant in the case of applications of the HARQ type II or III, sincethen each receiving station comprises one or more soft buffers, whereinthe contents of the soft buffer are soft-combined using the additionalretransmissions. By so doing, the channel number on which theretransmission is received implicitly indicates the soft buffer, thecontents of which have to be soft-combined with the data received viathis particular channel.

FIG. 5 shows a schematic representation of the transmission of new datapackets and retransmission for data packets from a transmitting stationvia four point-to-multipoint channels 21, 22, 23 and 24 according to anexemplary embodiment of the present invention as it may be implementedin the communication system of FIG. 1. The multicast channel used forthe primary or initial transmission of a new data packet is called firstchannel 21 or CH1. All four channels in the set are assumed to beslotted, in the sense that one data packet is carried within one slot.In a first step, data packet A is transmitted for the first time viafirst point-to-multipoint or multicast channel 21 as indicated byreference numeral 100. Then, the transmitting station continues to senda new data packet B via first multicast channel 21 (as indicated byreference numeral 200) and additionally sends the first retransmissionfor data packet A (A′) in parallel via multicast channel 22 (asindicated by reference numeral 101), to which only those receivingstations listen, which expect a retransmission for data packet A,because they failed to decode data packet A successfully. Thetransmitting station may be informed via a feedback channel by some orall of the receiving stations, which could not successfully decode thedata packet A, that they need a retransmission for data packet A.Similarly, if in the next slot, data packet B (and/or A taking intoaccount A′ possibly—by softcombining—together with A) cannot be decodedsuccessfully by at least one receiving station, they inform thetransmitting station about the need for a retransmission B′ for datapacket B and/or a second retransmission A″ for A, and hence B′ istransmitted via multicast channel 22 (as indicated by reference numeral201) and A″ is transmitted via multicast channel 23 (as indicated byreference numeral 102). At the same time, a new data packet C istransmitted via first multicast channel 21 (as indicated by referencenumeral 300). In a further step, a first retransmission C′ for datapacket C is transmitted and a second retransmission B″ for data packet Bhas to be sent. C′ is transmitted via multicast channel 22 (as indicatedby reference numeral 301) and B″ is transmitted via multicast channel 23(as indicated by reference numeral 202). On the other hand, data packetA has been successfully decoded by all receiving stations and thereforedoes not need to be retransmitted for a third time.

The gap between transmission of data packet C (reference numeral 300)and D (reference numeral 400) may e.g. be because no more data isavailable on the transmitting side at that point-in-time or for anotherreason. The gap could also be missing, i.e. transmission of data packetD would happen in the next slot after transmission of data packet C, andsimultaneously with the retransmissions C′ (reference numeral 301) andB″ (reference numeral 202).

From a different perspective, with the timing shown in FIG. 5, if thei-th retransmission carried on the (i+1)-th channel is sent in the y-thslot, where numbering of the slots of the first communication channel isthe same as for each communication channel of the plurality of secondcommunication channels, the initial transmission of this packet was donein the (y−i)-th channel. This relationship of slots and transmissions aswell as serial numbers of retransmissions for a data packet is part ofthe first and second switching scheme.

Depending on how long it takes until the transmitting station hasreceived feedback from some or all receiving stations, it can benecessary to delay the retransmission for a data packet. This is shownin FIG. 6, which assumes that it takes the duration of one slot afterthe reception of a data packet, until the transmitting station caninitiate a retransmission for this data packet, because it has receivedthe feedback concerning this packet from some or all receiving stations.Hence, the transmitting station sends data packets A (reference numeral100) and B (reference numeral 200) via the first channel CH1 (referencenumeral 21), which is used for new data. During the second slot (countedfrom the initial transmission of data packet A), in which packet B istransmitted from the transmitting station to the receiving stations, thetransmitting station receives the feedback concerning packet A from thereceiving stations, and finds out that a retransmission is required fordata packet A. This retransmission A′ (reference numeral 101) is theninitiated in the third slot and sent via the second channel CH2(reference numeral 22) used to convey the first retransmission, andsimultaneously with this, the new data packet C (reference numeral 300)is transmitted via CH1 (reference numeral 21). Accordingly, theretransmission B′ (reference numeral 201) for data packet B (referencenumeral 200) is sent in the fourth slot, after receiving the feedbackfor data packet B in the third slot. The feedback for the firstretransmission A′ of data packet A is then received during the fourthslot as well as the feedback for the retransmission C′, indicating thata second retransmission A″ for data packet A and a first retransmissionC′ for data packet C are needed. Simultaneously with an initialtransmission of a new data packet D (reference numeral 400) via CH1(reference numeral 21), the first retransmission C′ (reference numeral301) for data packet C is done via CH2 (reference numeral 22) and thesecond retransmission A″ for data packet A is done via CH3 (referencenumeral 23). In FIG. 6 it is assumed that the following newtransmissions of data packets D, and E (reference numerals 400 and 500)do not need retransmissions. For packet F and G (reference numerals 600and 700), FIG. 6 does not make an assumption, since they would have tocome later than the time that is covered by the figure.

The gap between transmission of data packet C (reference numeral 300)and D (reference numeral 400) may e.g. be because no more data isavailable on the transmitting side at that point-in-time or for anotherreason. The gap could also be missing, i.e. transmission of data packetD would happen in the next slot after transmission of data packet C, andsimultaneously with the retransmission B′ (reference numeral 201).

From a different perspective, with the timing shown in FIG. 6, if thei-th retransmission carried on the (i+1)-th channel is sent in y-thslot, where numbering of the slots of the first communication channel isthe same as for each communication channel of the plurality of secondcommunication channels, the initial transmission of this packet was donein the (y−2·i)-th channel, e.g. if the slot number on CH2 for the secondretransmission B″ (reference numeral 202) for B is 6, the initialtransmission of B is found on CH1 in slot (6−2·2)=2 (reference numeral200). This relationship of slots and transmissions as well as serialnumbers of retransmissions for a data packet is part of the first andsecond switching scheme.

For informing the transmitting station that A has now been successfullyreceived by all receiving stations, the known feedback scheme ACK/NACKmay be applied. Since in one slot several packets are received, multipleACKs/NACKs may have to be conveyed, one for each decoded multicastchannel in the multicast channel set. A straight forward implementationcould be providing in the direction to the transmitting station after aslot, in which data packets and retransmissions are received, as manyphases as there are multicast channels in the multicast channel set. Ineach phase, in a random access fashion, the receiving stations convey anindication whether the decoding on the channel, to which the phaserefers, resulted in an error or not.

As an alternative, one feedback message concerning the decoding resulton each decoded channel of the multicast channel set can be conveyedpreferably on a point-to-point channel which is exclusively assignedbetween the receiving stations and the transmitting stations. With suchfeedback, advantageously, retransmission may be stopped when allreceiving stations could decode the data packet.

In a further step a new data packet D is transmitted via multicastchannel 21 (as indicated by reference numeral 400) and data packets Band C are retransmitted for the third and second time respectively. Thesecond retransmission for data packet C (C″) is performed via channel 23(as indicated by reference numeral 302) and the third retransmission fordata packet B (B′″) is performed via multicast channel 24 (as indicatedby reference numeral 203). In a further step, data packet E istransmitted for the first time via first multicast channel 21 (asindicated by reference numeral 500). No additional transmission via theparallel multicast channels 22, 23, 24 is performed, since allpreviously transmitted data packets A, B, C and D have been successfullydecoded by each receiving station of the plurality of receivingstations.

The following rule may be applied for each data packet transmission:

Any initial or first transmission of a respective data packet isperformed via first multicast channel 21 and each retransmission for therespective data packet is carried out via a different multicastcommunication channel of a plurality of multicast communicationchannels, which does not include channel 21. The channel switchings inthe transmitting station and the receiving stations are defined in thefirst and second switching schemes, respectively.

If, at a certain time, a receiving station notices that it did notsuccessfully decode data packet B, it may switch to multicast channel 23and receive the second retransmission for data packet B (B″) in step 4,without the need for additional signaling.

Advantageously, the above described method of a point-to-multipoint setof channels implicitly contains the information on the maximum number ofretransmissions possible. Given that N+1 channels are defined in thechannel set of this scheme, each receiving station knows that afterreceiving N retransmissions of a data packet, no further retransmissionwill occur and it has to accept that this data packet will not bereceived error-free. In case of HARQ type II and III, the receivingstation may then flush the soft buffer for this packet, in order to usethe soft buffer for the next new packet to be transmitted.

Advantageously, the number of receiving stations does not influence thenumber of parallel multicast channels. Therefore, the above describedmethod has a major advantage over point-to-point transmission schemes,when a bigger number of receiving stations is involved, and eachreceiving station receives the multicast data via a point-to-pointchannel, i.e.—for each receiving station—via a channel, which only thisreceiving station decodes.

The physical channel used for the first retransmission is calledMC-ReTx-Ch 1 (multicast retransmission channel 1), that of the second iscalled MC-ReTx-Ch 2, that of the N-th transmission MC-ReTx-Ch N. Themulticast channel for the initial transmission of a new packet is calledMC-Ch. All channels in the set are assumed to be slotted in the sensethat within one slot, one packet is carried.

After the first transmission of packet A to a plurality of recipients,for example, ten recipients, four still need a retransmission tocorrectly decode packet A. To organize the retransmissions, as may betaken from FIG. 5, any initial transmission of a data packet is sent viaMC-Ch and the i-th retransmission for a packet is carried out viaMC-ReTx-Ch (i), i.e. the channel for the i-th and the (i+1)-thretransmission is different.

The shifting of the channels to carry the retransmissions may berequired for reasons of avoiding signaling, which indicates which of thechannels in the channel set is a new packet, and which carries the i-thretransmission for a packet previously sent (i=1, 2, . . . , N).

Advantageously, in the concept described with reference to FIG. 5 andFIG. 6, the point-to-multipoint channel set implicitly containsinformation with respect to the maximum number of retransmissionspossible: Given that N+1 channels are defined in the channel set of thisscheme, each recipient or receiving station knows that after receiving Nretransmissions for a packet (and this retransmission was carried out onMC-ReTx-Ch N), no further retransmission will take place, and it hasthus to accept that this packet will not be received error-free, unlessthe transmitting station decides based on the feedback from thereceiving stations, to initiate a new initial or primary transmission ofthe otherwise lost packet via MC-Ch. This may change the sequence of thedata packets in the receiving stations, and hence a sequence number isrequired, which is carried in each data packet. Based on this sequencenumber, the receiving station can reorder the received packets toreconstruct their original sequence, as described e.g. in 3GPP TS 25.321“3rd Generation Partnership Project: Technical Specification Group RadioAccess Network; Medium Access Control (MAC) Protocol Specification(Release 5) Version 5.5.0” which is hereby incorporated by reference. Inother words, as long as retransmissions are only sent via the multicastcommunication channels of the plurality of second multicastcommunication channels such sequence numbers are not needed.

In order to assess the advantages of the described invention, thefollowing scheme 1, which comprises one exemplary embodiment of theinvention, is considered together with scheme 2, in which the same datadestined to N recipients is transmitted to the recipients via Npoint-to-point channels:

Scheme 1 (point-to-multipoint channel set): without any sophisticatedstatistical multiplexing scheme, it may be required to allocate Nparallel multicast channels in order to be able to send Nretransmissions, in addition to the multicast channel used for theinitial transmissions of a data packet. Each recipient would need afeedback channel, via which it may transmit an acknowledgement or anegative acknowledgement indicating the request for a retransmission.The number of recipients does not influence the number of parallelmulticast channels, but, of course, there are as many feedback channelsrequired as there are recipients.

Scheme 2 (only using point-to-point channels): the multicast informationis sent via point-to-point channels to the recipients, i.e. receivingstations, which requires as many point-to-point channels as there arerecipients. Since the number of retransmissions does not influence thenumber of channels required, the number of feedback channels is the sameif the concept of a point-to-multipoint channel set is applied.

Whether scheme 1 or 2 is more efficient from a channel resourcepoint-of-view depends on the number of receiving stations and the numberof retransmissions. For example, if there are only two recipients, twopoint-to-point channels would be sufficient without any limit for themaximum number of retransmissions. Obviously, if the maximum number ofretransmissions is the same as the number of recipients, those schemesare equally efficient in terms of channel resources. If the maximumrequired number of retransmissions N_(ReTxMAX) is small (such athreshold for the maximum number of retransmissions depends, of course,on the channel conditions), and the number of N_(recipients) ofrecipients is larger than the number of retransmissions, the concept ofthe point-to-multipoint channel set has a clear advantage in terms ofchannel resources. Thus, with ten recipients and a maximum of fourretransmissions, four multicast channels would be required to allow fora retransmission protocol, while it would be ten if point-to-pointchannels were used.

Apart from the fact that a channel capacity reduction may be achieved bythe concept of the point-to-multipoint channel set, ifN_(ReTxMAX)<N_(recipients), the concept also provides a delay reduction,since the retransmissions are carried out simultaneously with the newtransmissions: assuming (as above) ten recipients and N_(ReTxMAX)=4, allpackets are received with a maximum delay of four slots. Ifpoint-to-point channels (scheme 2) are used, assuming alsoN_(ReTxMAX)=4, the overall delay may, in many cases, be higher, becausewhen a packet needs a retransmission, the channel is occupied andsubsequent packages have to wait. If the first packet needs fourretransmissions, all subsequent packets are delayed by the four slots,if the second packet needs three transmissions, all subsequent packetsare delayed by 4+3 slots. This may be particularly important if thepackets have to be reassembled on the receiving side into a service dataunit (hereinafter as usual referred to as “SDU”), which is destined forthe upper layer. The SDU can only be reassembled if all its segments arereceived error-free. From such a point of view, it may also be feasibleto apply point-to-multipoint transmission for real time services, if themaximum number of retransmissions is not too large.

According to a further aspect of the above exemplary embodiment of thepresent invention, in a wireless environment where receiving stationsface different channel conditions in a way that, for example, thosewhich are close to the base station receive very good signal strength,while those at cell edges have to cope with rather weak signals, apartitioning of the set of recipients may be particularly advantageous.Thus, the plurality of receiving stations within reach of the basestation may be partitioned into groups of receiving stations havingcomparable or similar channel conditions. Comparable or similar channelconditions may relate to comparable signal strengths or to a comparabletransmission delay, or to comparable interferences in the channels orelsewhere. Then, the point-to-multipoint data packet transmission may beperformed by the base station (or transmitting station) for each of thegroups individually. Advantageously, by grouping the receiving stationsinto groups, for which the maximum number of retransmissions is expectedto be the same, since they have the same or comparable channelconditions, a highly efficient data transmission may be provided.

As already indicated above, the present invention may also be applied inthe context of 3GPP (“3rd Generation Partnership Project”) FDD modeFrequency Division Duplex mode).

The 3GPP FDD mode provides an HS-DSCH (“High Speed Downlink SharedChannel”), which may be used with code multiplexing. By this, within oneslot, it is possible to transmit data to the same user, using severalcodes. Instead of using several codes to increase the data rate withinone slot, each code channel or subsets of code channels of all codesavailable for one user in a slot may be used as one of the channels of apoint-to-multipoint channel set, as described above. By applying HARQtype II and/or III, the retransmissions may carry only incrementalredundancy, so that the interference generated by additional codechannels used for the retransmissions becomes smaller than that of anadditional code channel, which carries self-decodable redundancy. Inaddition, the spreading factor on the code channel used for an initialtransmission in parallel to retransmissions for earlier packets ondifferent parallel code channels could be increased so that for thisinitial transmission a smaller packet is carried in order to furtherreduce the overall interference resulting from the new transmission inparallel to retransmissions for earlier packets, which interferenceimpacts neighboring cells and is generated by the cell, in which thisHS-DSCH is modified so that point-to-multipoint transmission ispossible.

In another transmission system with broadcast behavior, or wirelesstransmission system, or cellular mobile communication system, sending ondifferent channels a retransmission for a packet in parallel orsimultaneously with a new transmission or retransmissions of earlierpackets, can also be performed by means of frequency division multipleaccess (FDMA), i.e. that for the initial transmission of a data packetalways a first frequency channel is used, for the first retransmissionfor a data packet always a second frequency channel is used, for thesecond retransmission for a data packet, always a third frequencychannel is used, etc.

1. A method for performing point-to-multipoint data transmission from atransmitting station to a plurality of receiving stations, the methodcomprising the steps of: transmitting data from the transmitting stationto the plurality of receiving stations via a first communicationchannel; performing a first channel switching in the transmittingstation when decoding of the transmitted data fails at a receivingstation of the plurality of receiving stations, from the firstcommunication channel to a second communication channel; andretransmitting the data via the second communication channel while thefirst communication channel remains in use for transmitting new data:wherein multiple retransmissions automatically occur on a plurality ofdifferent predetermined channels until the data is successfullytransmitted via one of the plurality of different predeterminedchannels, and wherein additional data is simultaneously transmitted onthe first communication channel during the multiple retransmissions; andwherein after the decoding of the transmitted data fails at thereceiving station, the receiving station automatically establishes aconnection for retransmission on a channel of the plurality ofpredetermined channels without causing notification messaging to occurbetween the receiving station and the transmitting station.
 2. Themethod of claim 1, wherein the data is transmitted as a data packet, themethod further comprising the steps of: performing a second channelswitching in at least one second receiving station of the plurality ofreceiving stations from the first communication channel to the secondcommunication channel; wherein the first channel switching is performedin accordance with a first switching scheme; wherein the second channelswitching is performed in accordance with a second switching scheme; andwherein the first switching scheme corresponds to the second switchingscheme.
 3. The method of claim 2, wherein a predefined delay timeinterval exists between the transmission of the data packet and theretransmission of the data packet; and wherein the delay time intervalis set forth in the first and second switching schemes.
 4. The method ofclaim 2, wherein transmission of original data packets is carried out onthe first communication channel.
 5. The method of claim 2, wherein, whena receiving station of the plurality of receiving stations erroneouslydecodes the data packet, the receiving station of the plurality ofreceiving stations sends a negative acknowledgement message to thetransmitting station via a third communication channel; and wherein,when the transmitting station receives no negative acknowledgementmessage, no retransmission of the data packet is performed by thetransmitting station.
 6. The method of claim 2, wherein a plurality ofsecond communication channels are provided, wherein a plurality ofretransmissions of the data packet are performed via the plurality ofsecond communication channels; and wherein each of the retransmissionsof the plurality of retransmissions are carried out via a differentcommunication channel of the plurality of second communication channels,in accordance with the first and second switching schemes.
 7. The methodof claim 2, wherein a plurality of second communication channels areprovided, wherein a plurality of retransmissions of the data packet areperformed via the plurality of second communication channels; whereineach of the retransmissions of the plurality of retransmissions is arecarried out via a different communication channel of the plurality ofsecond communication channels, in accordance with the first and secondswitching schemes; and wherein retransmissions of different data packetsare performed simultaneously via different communication channels of theplurality of second communication channels, and simultaneously with thetransmission of additional data packets via the first communicationchannel.
 8. The method of claim 2, wherein the plurality of receivingstations in close proximity to the transmitting station are partitionedinto groups of receiving stations with comparable channel conditions ofthe first and second communication channels; wherein thepoint-to-multipoint data packet transmission is performed for each ofthe groups individually.
 9. The method of claim 2, wherein transmissionand retransmission of the data is performed via a wireless communicationlink.
 10. The method of claim 2, wherein the method is an extension of aHybrid Automatic Repeat Request (HARQ) protocol in a Universal MobileTelecommunications System (UMTS).
 11. The method of claim 2, wherein themethod is applied in a 3^(rd) Generation Partnership Project FrequencyDivision Duplex (3GPP FDD) mode; and wherein code channels provided inthe 3 GPP FDD mode are used as at least one of the first and secondcommunication channels.
 12. A communication system for performing apoint-to-multipoint data transmission from a transmitting station to aplurality of receiving stations, wherein the transmitting station isadapted to transmit data from the transmitting station to the pluralityof receiving stations via a first communication channel; wherein thetransmitting station is adapted to automatically perform a first channelswitching from the first communication channel to a second communicationchannel when decoding of the transmitted data fails at a receivingstation of the plurality of receiving stations; wherein at least one ofthe plurality of receiving stations is adapted to perform a secondchannel switching from the first communication channel to the secondcommunication channel; wherein the transmitting station is adapted toretransmit the data via the second communication channel while the firstcommunication channel remains in use for transmitting new data; andwherein multiple retransmissions automatically occur on a plurality ofdifferent predetermined channels until the data is successfullytransmitted via one of the plurality of different predeterminedchannels, and wherein additional data is simultaneously transmitted onthe first communication channel during the multiple retransmissions; andwherein after the decoding of the transmitted data fails at thereceiving station, the receiving station automatically establishes aconnection for retransmission on a channel of the plurality ofpredetermined channels without causing notification messaging to occurbetween the receiving station and the transmitting station.
 13. Atransmitting station for a communication system for a performingpoint-to multipoint data transmission to a plurality of receivingstations, wherein the transmitting station is adapted to transmit datafrom the transmitting station to the plurality of receiving stations viaa first communication channel; wherein the transmitting station isadapted to perform a channel switching from the first communicationchannel to a second communication channel when decoding of thetransmitted data fails at a receiving station of the plurality ofreceiving stations; wherein the transmitting station is adapted toretransmit the data via the second communication channel while the firstcommunication channel remains in use for transmitting new data; andwherein multiple retransmissions automatically occur on a plurality ofdifferent predetermined channels until the data is successfullytransmitted via one of the plurality of different predeterminedchannels, and wherein additional data is simultaneously transmitted onthe first communication channel during the multiple retransmissions: andwherein after the decoding of the transmitted data fails at thereceiving station, the receiving station automatically establishes aconnection for retransmission on a channel of the plurality ofpredetermined channels without causing notification messaging to occurbetween the receiving station and the transmitting station.
 14. Areceiving station suitable to use with a communication system forreceiving a data packet transmitted from a transmitter to a plurality ofreceiving stations by means of a point-to multipoint data transmission,wherein the receiving station is adapted to receive the data transmittedvia the first communication channel; wherein the receiving station isadapted to perform a channel switching from a first communicationchannel to a second communication channel when decoding of thetransmitted data fails at a receiving station of the plurality ofreceiving stations; wherein the receiving station is adapted to receivethe retransmitted data via the second communication channel while thefirst communication channel remains in use for transmitting new data;wherein multiple retransmissions automatically occur on a plurality ofdifferent predetermined channels until the data is successfullytransmitted via one of the plurality of different predeterminedchannels, and wherein additional data is simultaneously transmitted onthe first communication channel during the multiple retransmissions; andwherein after the decoding of the transmitted data fails at thereceiving station, the receiving station automatically establishes aconnection for retransmission on a channel of the plurality ofpredetermined channels without causing notification messaging to occurbetween the receiving station and the transmitting station.